A lens or spectacle lens generally has a front side and a rear side which are both bordered by an edge. The edge comprises an edge surface which is usually designed according to the frame. The optical effect of the front side and the optical effect of the rear side overlap and form the overall optical effect or optical property. The front side or the rear side here form the first side, while the rear side or the front side forms the opposite second side.
During the manufacture of lenses, in particular, spectacle lenses made of synthetic material, a semi-finished lens product is usually used, the first side of which is pre-finished, i.e. the form of the surface of the first side is produced according to a pre-specified form. In general, this is a cast moulded part, wherein the surface of the first side corresponds to a form of the form shell. In this case, the form shell comprises the pre-specified form to be formed. Due to the material properties of the synthetic material used, and due to the cooling process after injection, deviations occur between the pre-specified form of the tool and the actual form of the surface of the first side which is in reality attained.
Ultimately, the method permits the detection of faulty semi-finished parts which are erroneously fed into the process. A time-intensive and expensive machining of a faulty semi-finished product or blank is avoided, in particular, the relatively expensive processing stages such as milling, turning, polishing, hardening and coating.
The semi-finished product can be milled with a corresponding form of the surface of the first side. In this case, the deviations between the pre-specified surface and the actual surface which is produced in reality are generally lower, but also present.
These deviations have to date not been taken into account when estimating the pre-specified form of the surface of the second side of the lens to be manufactured. When estimating the pre-specified form of the surface of the second side, only the pre-specified form of the surface of the first side was taken into account.
A deflectometric stereo method for determining the local height and the local surface standard of reflective surfaces such as aspherical lenses is known from DE 10 2004 020 419 B3. Here, full reference is made to the principle relating to the measurement method, which for persons skilled in the art can be derived from DE 10 2004 020 419 B3.